Light emitting diodes (LEDs) are widely available, inexpensive, and efficient light sources. For uses such as sport headlamps, one or two state of the art LEDs provide adequate light. While simple light distributions of rotational symmetry are sufficient for low quality products or less demanding uses, more complex light distributions are being employed for better vision when walking, running, or cycling with a headlamp. It is beneficial to produce a relatively narrow hotspot of typically 10° full width at half maximum (FHWM) so to illuminate objects far away from the user, while a lower level intensity background light is needed to provide lighting of the ground close to the user. Such a background light is not needed upwards from the hotspot so that a background beam that is tilted relative to the hotspot beam is beneficial.
A collimator configuration as seen in FIG. 1 is used in a current headlamp product made by Silva Sweden AB that produces a wide beam off-axis background light around a narrow intense on-axis hotspot. The lens 100 is of a type herein called a “photon funnel” that has a central “collimator cavity” containing the light source. The wall of the cavity has a front or center lens 103 and a side or peripheral cavity surface. In a center section of the photon funnel, light passes by refraction through the center lens 103 and an exit surface (which in FIG. 1 is part of a front surface 104), while the majority of the light passes through the cavity side surface by refraction, is reflected by total internal reflection (TIR) at a back surface 102, and exits through the front surface 104 by refraction.
In the Silva product, the center lens 103 of lens 100 is a rotationally symmetric surface that has its rotational axis tilted with respect to the light source axis to provide an off-axis background light while surface 102 collimates the majority of the light from the LED to form a narrow hot spot. This architecture works well, if the amount of light that is needed for the background illumination is roughly one third of the full light emitted by a Lambertian LED, as this is the typical amount of light collected by the center lens of a conventional photon funnel. If more or less light is wanted in the off-axis beam, this configuration cannot be used. Moreover the center lens provides a relatively wide beam by nature of the lens 103, so that if a narrow off-axis beam is wanted, the center lens cannot provide such beam.
In all of the described embodiments, the cavity side surface is a surface of rotation about a center axis, and the light source is an LED chip centered on and coaxial with the center axis of the cavity. A typical LED chip is flat, and is a Lambertian emitter with its emission symmetrical about an axis perpendicular to the flat chip. The LED chip thus typically has a well-defined central axis. In the present specification, the terms “on-axis” and “off-axis” are used here with respect to the common center axis of the collimator cavity and the LED chip. In all of the embodiments, one of the hotspot beam and the background beam is directed along the center axis, and the other beam is directed along a second axis, referred to as a “tilted axis,” diverging from the center axis. In all of the embodiments, the exit surface of the optics is flat, and the surface normal of the exit surface coincides with the cavity center axis. However, exit surfaces of other shapes and orientations can be implemented.
The head lamp itself often provides means to adjust the direction of light emission of the entire lamp, so that the narrow beam can be adjusted for far vision while the wide beam will provide near vision. Thus, as will be shown below with reference to FIG. 4, the same functionality as in the Silva lamp can be achieved by the “dual” case in which a tilted center lens provides a hotspot beam along the tilted axis, and an on-axis reflector provides an on-axis background beam. However, the simple dual configuration will then typically direct two-thirds of the light into the background beam and one-third into the hotspot beam, which may not be optimal.
Other applications besides sport headlamps of partially or fully off-axis LED collimators would be in architectural lighting to create certain lighting effects, such as illuminating a wall from a lighting fixture that is oriented parallel to the wall, in street lighting, and many other applications.